1. Field of the Invention
The present invention relates generally to semiconductor integrated circuits, and more specifically to a method for forming polycrystalline silicon contacts in semiconductor circuits.
2. Description of the Prior Art
In semiconductor integrated circuit processing, electrical connections are made between various levels of interconnect and between electrical interconnect levels and a monocrystalline substrate. Electrical quality of these contacts is important to the quality and reliability of integrated circuits fabricated using them.
Damage to the monocrystalline silicon lattice in the contact area can degrade the quality of the contact. Such damage is often seen after reactive ion etching and ion implant processing steps. This damage is caused by the high energy particle bombardment of the surface, and is manifested as crystalline lattice defects and surface irregularities.
Defects at the silicon surface cause oxidation problems when a layer of polycrystalline silicon is deposited over the contact opening. The silicon dioxide insulator formed at the contacts results in a poor electrical connection with high resistance. Electrical quality of the contacts often varies from lot to lot, and even varies between individual chips within a single semiconductor wafer.
The quality of electrical contacts can vary from lot to lot depending on the amount of etching or ion implant performed. For example, a slight over etch used to fully clear oxide from a contact opening can greatly increase the damage to the underlying silicon layer. This in turn greatly decreases contact quality, because the amount of later oxidation seen at the surface depends on the extent of damage caused by high energy ions. Unavoidable process variations in the amount of etching or ion implantation performed during integrated circuit fabrication, and in corresponding variances in the amount of oxide formed at the interface, means that the actual contact resistance is not well controlled.
In many process flows, an annealing step is performed prior to the deposition of the polycrystalline silicon interconnect layer. This annealing step reduces the surface defects somewhat, improving the oxidation problem in the contact. However, the contact quality is still not as good as often desired. In addition, contact quality still varies depending upon variance in process parameters.
It would therefore be desirable to provide a method for semiconductor integrated circuit processing which provides improved polysilicon to silicon contacts.